The combustion chambers with which we are concerned here are of the tubular shell variety. In order to better withstand the corrosive effects of combustion gases, these combustion chambers are very often made of stainless steel which material has a relatively large coefficient of thermal expansion. Accordingly, the combustion chambers are prone to undergo longitudinal and radial expansions and contractions as the heater is turned on and off to meet the demands of the space being heated.
The combustion chamber is usually supported at its rear end through a sheet header which also communicates with heat exchange tubes so that hot combustion products produced in the combustion chamber by the burner can pass from the combustion chamber through the header and heat exchange tubes to the flue. Fresh air, on the other hand, is drawn into the heater, is circulated around the combustion chamber and heat exchange tubes to heat it and then the heated air is discharged into the space directly or by way of ducts. It is essential then that the joint between the rear end of the combustion chamber and the header be completely fluid-tight so that no combustion products can be entrained in the air being expelled by the heater into the living space.
Since the headers and fire tubes are subjected to lower temperatures than the combustion chamber, it is customary to construct them out of a material other than stainless steel, carbon steel for example, which has a different coefficient of thermal expansion. Consequently, when the heater cycles between its high and low temperature extremes, the combustion chamber and the headers expand by different amounts. For that reason, it has proven difficult to provide a fluid-tight joint between the combustion chamber and the rear header which will maintain its integrity despite repeated temperature cyclings of the heater.
One technique for solving this problem is disclosed in U.S. Pat. No. 2,984,235. It involves the providing of a ring around the rear end of the combustion chamber, which ring is welded to the rear header sheet and to the combustion chamber wall at spaced-apart locations such that during thermal cyclings of the heater, the ring is able to roll to relieve stresses on the circular welds caused by the differential expansion and contraction movements of the header and the chamber. In another prior construction, the ring is shrunk fit onto the chamber and a single weld bead connects the ring to the rear header sheet. Both those conventional joints have a serious disadvantage, however, in that the end of the combustion chamber extends through the header sheet and cannot reject heat efficiently to the cooler surfaces of the header sheet. Resultantly, it is heated excessively causing high stresses which give rise to potential failures of the fluid-tight joint between the chamber and header which can shorten the useful life of the heat exchanger as a whole.
Also, because the ring welded between the combustion chamber and the header is positioned on the side of the header sheet away from the chamber, care must be taken to ensure that the chamber cannot pull out of the header sheet in the event of excessive lengthwise contraction of the chamber relative to the header. In the past, this has been accomplished by fitting ring segments having a square cross-section around the end of the chamber on the opposite side of the rear header sheet from the round ring and welding the square ring segments to the end of the chamber. The segments act as abutments to minimize any likelihood of the combustion chamber pulling out of the rear header sheet.
In addition to that, a series of metal straps whose opposite ends are connected to the rear header sheet and to the combustion chamber, respectively, are distributed around the joint between those two elements to further eliminate the possibility of the chamber pulling out of the header and falling down into the heater. While those safety precautions have eliminated the problem of combustion chamber pull-out at the rear end of the chamber, they require a relatively large number of cutting and welding steps.
The seal between the forward, cooler end of the combustion chamber and its support has not been totally satisfactory either because some fluid leakage still occurs from the interior of the heater into the space being heated because of leakage past that seal, when high static air pressures are maintained inside the heater.